I. Field of the Invention
This disclosure relates generally to apparatus and methods for location estimation. More particularly, the disclosure relates to providing calibrated sensor measurements in a building or other structure when satellite navigation systems are unavailable or inadequate.
II. Background
Today, mobile devices having an ability to provide position estimates while in open areas are ubiquitous. In rural environment with a view of four or more positioning satellites, a mobile device acquires satellite signals and computes a position estimate based solely on these satellite signals. When in an urban environment or indoors, however, sufficient satellite signals are usually unobtainable. When adequate satellite signals are inaccessible, mobile devices often use signals from a cellular network to obtain a rough position estimate (e.g., via Cell IDs, trilateration, or triangulation). To improve position estimates when satellite and cellular signals are insufficient, a mobile device may use inertial sensors and dead reckoning. Over a long period, however, inertial sensors and conventional dead reckoning accumulate errors resulting in unreliable position estimates.
FIGS. 1 and 2 show an overhead view of a person with relation to a building. In FIG. 1, a person at a current location (marked with an ‘x’) walks along a path 120 with a mobile device 100 outside of three buildings 110. The Cardinal direction North 200 is shown in this overhead view as pointing up. The buildings 110 in this example complex are shown parallel or square with an exterior angle 210 (θEXTERIOR) in line with a primary exterior wall of the buildings 110. That is, each exterior wall is parallel or perpendicular to the exterior angle 210 (θEXTERIOR), which is shown at an angle (θN) offset from North 200. While outside, the mobile device 100 is able to determine a position estimate from viewable satellites and/or cellular signals. Once inside, however, the mobile device 100 may not be able to receive adequate signals from either satellites or cellular base stations.
FIG. 2 shows a floor plan of an example building 110. The mobile device 100 continues along path 120 into building 110 and is shown currently in a hallway 130. Deep inside building 110, the mobile device 100 is unable to acquire satellite signals or perform satellite-based positioning or cellular-based positioning. In this situation, the mobile device 100 uses its sensor 330 (i.e., one or more accelerometer(s) 334 and/or gyrometer(s) 336 and/or magnetometer 338) to perform dead reckoning without a need for any signals from satellites or base stations. A gyrometer, such as gyrometer(s) 336, includes a gyroscope. Dead reckoning, however, accumulates error with each position estimate because of calibration errors in the sensor 330. Over time, such position estimates and estimated headings can accumulate so much error or drift that, instead of properly placing a mobile device 100 at a particular location, such estimates may improperly place the mobile device 100 in a different room or hallway, or even outside building 110 altogether. Therefore, dead reckoning and other applications using sensors 330 will benefit if the sensor 330 are kept well calibrated.